1. Field of the Invention
This invention relates to a liquid level sensing apparatus for sensing the liquid level of liquid including serum in automatic chemical analysis.
2. Description of the Related Art
A well-known liquid level sensing apparatus used for an automatic chemical analyzer is disclosed in United States patent application Ser. No. 028,705 (filing data Mar. 20, 1987). This analyzer includes a bridge circuit, which generates a signal corresponding to a vertical displacement of a pipette for extracting a sample, a reagent, etc., when such movement is caused. The probe is connected as an element of the bridge circuit, and the bridge circuit generates an output signal corresponding to a change in the impedance between the probe and liquid level of sample accommodated in a sample container as the probe approaches the liquid level of the sample. When a CPU detects this signal, it provides a zero-setting signal corresponding to the input signal to an automatic phase controller. The automatic phase controller controls the phase of a reference signal from an oscillator according to the zero-setting signal to hold a phase difference of 90 degrees between the reference and input signals.
With this prior art liquid level sensing apparatus, the electrostatic capacitance between the pipette and liquid level is instable and varies due to vibrations of the pipette or the like stemming from the looseness of the mechanism of the analyzer for a predetermined period of time from the start of descent of the pipette. For this reason, the liquid level sensor detects the liquid level erroneously and produces an erroneous detection signal.
Further, the impedance between the pipette and sample surface is capacitive or inductive according to the kind of the sample. For example, if the sample is serum, the impedance is capacitive. With pure water, the impedance is inductive. With the variation of the detected impedance between capacitive and inductive impedances, negative and positive components corresponding to the capacitance and inductance, respectively, are generated as liquid level detection signals. When such opposite-phase detection signals are generated randomly, the liquid level is detected erroneously.